1. Field of the Invention
The present invention relates to a method of desulfurizing pig-iron melts and a composition used for such desulfurization.
2. Discussion of Prior Art
The increasing contents of sulfur in ores and other materials used in pig iron production, e.g., coal and coke make it absolutely essential to desulfurize pig iron outside the blast furnace. A wide range of materials and methods that lead to satisfactory levels of desulfurization are known. Mixtures based on calcium carbide and diamide lime (U.S. Pat. No. 3,598,573) are preferred. Compounds containing calcium in combination with hydrocarbons like oil or paraffin (French Patent 1,166,389 and U.S. Pat. No. 2,863,755) have also been proposed, German Auslegeschrift 2,531,047 proposes a method of desulfurizing pig iron that employs a desulfurizing agent containing calcium carbide, calcium cyanamide or lime plus 0.5 to 3.5% of powdered aluminum or magnesium in terms of the calcium compounds. U.S. Pat. No. 4,049,442 discloses desulfurizing with a mixture of calcium carbide and diamide lime wherein the content of the diamide lime is from 40 to 70% by weight of the desulfurizing composition. U.S. Pat. No. 4,154,606 discloses a desulfurization agent containing at least one alkaline earth metal carbonate and at least one reducing metal carbide and optionally a reducing metal or an alloy thereof.
The aforesaid desulfurizing agents, especially those based on calcium carbide in combination with diamide lime or calcium carbonate, have been introduced into the art and are manufactured in large quantities and employed in the iron and steel industry.
The gas-releasing components are intended to distribute the finely milled particles of desulfurizing agent throughout the melt. The release of carbon dioxide from carbonate constituents, however, can under the conditions prevailing in the pig-iron melts, lead to oxidation processes as a result of dissociation into carbon monoxide and oxygen. Part of the desulfurization-active compounds are accordingly lost to the desulfurizing reaction. Hence, the efficiency of the desulfurant in terms of percent sulfur reduction per unit weight of desulfurant per unit weight hot metal is in need of improvement.
Even the desulfurization employing mixtures of lime and magnesium described in U.S. Pat. Nos. 3,998,625 and 4,266,969 entails the drawback of large slag volumes due to the practical lack of a desulfurizing effect on the part of the lime.
The problem accordingly exists of providing a method of desulfurizing pig iron that eliminates or reduces these drawbacks and which permits use of short desulfurization times while keeping slag generation at a low level.